This invention relates to analog to digital conversion and more particularly to a logarithmic analog to digital conversion circuit in which an input alternating current (AC) signal to be converted to a digital signal is sequentially compared to a locally generated reference signal. When the attenuated reference signal equals the input signal, the amount of attenuation is used to generate a digital representation of the input analog signal.
Analog to digital (A/D) converters are well known and generally apply successive approximation or time constant measurement techniques to direct current (DC) signals to create a digital representation of the input analog signal. This representation is typically a binary word having a number of bits corresponding to the deired accuracy of the digital conversion.
If the input signal has a large range of signal strength (large dynamic range), the number of bits in the binary word must be correspondingly large. Furthermore, since the same number of bits are available to represent both the largest input signal strength and the smallest signal strength, the error represented by the difference of a least significant bit zero to a least significant bit one is proportionally smaller for the largest input signal strength than for the smallest input signal strength. To remedy this error, logarithmic analog to digital converters have been used.
Logarithmic A/D converters enable analog signals having signal strengths varying over many orders of magnitude to be converted to digital words. The least significant bit error, then, becomes a specified percentage of the input signal. Logarithmic A/D converters, however, may have absolute errors in magnitude which are caused by amplifier and detector drift with temperature and time. This problem becomes more acute when the input signal is not direct current but an alternating current of reasonably high frequency.
Therefore, one object of the present invention is to provide an analog to digital converter which converts a wide dynamic range alternating current into a digital format.
It is another object of the present invention to provide the means for compensating the analog to digital converter for the effects of temperature and time.
It is a further object of the present invention to rapidly converge on the digital representation of the analog alternating current signal.